DNA polymerases are enzymes that can synthesize new DNA strands along template DNA strands in vitro. DNA polymerases can synthesize new DNA strands from a template DNA, an oligonucleotide serving as a primer, and four types of deoxynucleotides (dATP, dGTP, dCTP, and dTTP). DNA polymerases are also used in many genetic engineering techniques, including nucleotide sequencing and PCR.
Thermostability of polymerases is essential for PCR, and the current protocol of nucleotide sequencing generally uses the cycle sequencing method using a thermostable DNA polymerase as a standard technique. In order to find a thermostable enzyme, one would usually search enzymes produced by thermophilic microorganisms. Among thermophilic bacteria, those which proliferate at an optimum growth temperature of at least 80° C. are particularly referred to as “hyperthermophilic bacteria” and serve as excellent resources for thermostable enzymes. Taq DNA polymerases (also referred to as “Taq polymerases”) which are currently widely used in PCR were originally isolated from the thermophilic eubacterium Thermus aquaticus. 
Based on the similarity in amino acid sequences, DNA polymerases are categorized into seven groups: Families A, B, C, D, E, X and Y. Enzymes belonging to the same family basically exhibit very similar properties. The enzymes that are in practical use are those belonging to Families A and B.
Family A enzymes have superior performance in recognizing dideoxynucleotides as substrates and are most appropriate for nucleotide sequencing. Thus, the enzymes contained in currently commercially available sequencing kits are all those which belong to Family A and are derived from thermophilic eubacteria. In PCR, Family A and B enzymes are selectively used depending on the purpose.
Family B enzymes are not suitable for nucleotide sequencing because of poor incorporation of dideoxynucleotides but have 3′-5′ exonuclease activity which is involved in the accuracy in synthesizing DNA strands according to the sequences of template strands—during amplification, the enzymes of this family produce less errors than Family A enzymes such as Taq polymerases with no exonuclease activity. The Family B enzymes that are commercialized are those derived from hyperthermophilic archaea. In order to perform PCR more accurately, it is advisable to use Family B enzymes, whereas in order to amplify long-chain DNA, Family A enzymes can be selected due to their superior extensibility and superior DNA synthesis efficiency.
Comparison between the two DNA polymerases that are derived from bacteria belonging to the genus Thermus and which have been up to now widely used as PCR enzymes shows that Taq DNA polymerase only has weak reverse transcriptional activity, while Tth DNA polymerase (“Tth polymerase”) derived from Thermus thermophilus has significantly strong reverse transcriptional activity. This property of Tth polymerase is utilized in a simple RT-PCR technology in which a single enzyme is used in a single reaction tube to synthesize cDNA from mRNA by reverse transcription and then amplify the synthesized cDNA. Since the optimum temperature of this enzyme is high, the enzyme makes it possible to perform a reverse transcription reaction at relatively high temperatures (around 60° C.) and is also effective for the reverse transcription of RNA which easily forms a three-dimensional structure, but the enzyme is not suitable for the synthesis of long cDNAs like those reaching as long as several kilo bases in length.
PCR is a gene analysis technology that is widely used throughout the world as a routinely utilized technique. Accordingly, there is a need for a DNA polymerase that is more convenient, easier-to-use, and more reliable, and it is also desired to provide various DNA polymerases that can amplify various DNAs appropriately depending on the template to be used as well as the purpose to be needed for PCR such as extensibility, rapidity, and accuracy.
As regards modification of Taq polymerases, there have been hitherto reports stating that primers were designed based on the segments of an amino acid sequence highly conserved in Family A DNA polymerases, each of which contains an active site, gene fragments were amplified by PCR using DNA samples derived from hot spring soil as templates, and the corresponding segments of a wild-type Taq polymerase gene were substituted by the amplified fragments, whereby obtained were chimeric DNA polymerases with higher extension activity than Taq polymerases (Patent Documents 1 and 2). Another report showed that on the basis of metagenomic analysis and the three-dimensional structure information of DNA polymerases, one or more mutations were introduced that produce an increased the total electric charges of glutamic acid at position 742 and alanine at position 743 in an amino acid sequence of a Taq polymerase, whereby obtained was a modified Taq polymerase that is superior to the Taq polymerase in at least one of: primer extension activity; binding activity on a primer annealed to a template DNA; and PCR performance (Patent Document 3).